1. Field of the Invention
The present invention relates to a solder ball printing apparatus and a solder ball printing method in which solder balls are printed on electrodes formed on a surface of a substrate.
2. Description of the Related Art
In the case where solder balls are disposed on a mask to fill opening portions of the mask with the solder balls in a conventional solder ball printing apparatus, the magnetic mask is fixed while keeping a gap between the mask and a substrate constant. Therefore, protruding posts are provided on the back surface of the mask, and magnets are provided on the table side across the substrate.
An example of such a solder ball printing apparatus is disclosed in Japanese Patent Application Laid-Open No. 2008-192818. In order to accurately and reliably mount conductive balls on terminal areas of a substrate, the device described in Japanese Patent Application Laid-Open No. 2008-192818 has a backup plate on which the substrate is mounted to release the terminal areas, and a metal mask. Further, the device includes a feeding mask in which plural through-holes corresponding to the plural terminal areas of the substrate are formed, a fixing block that fixes the ends of the feeding mask so that the feeding mask faces one surface of the substrate, and magnets that attract the feeding mask to the backup plate side with a magnetic force. In addition, the attraction force of the magnets is smaller in the middle portion of the feeding mask than in the peripheral portion.
Another example of a conventional solder ball printing apparatus is disclosed in Japanese Patent Application Laid-Open No. 2012-19000. In the solder paste printing apparatus described in Japanese Patent Application Laid-Open No. 2012-19000, highly-accurate printing is performed to prevent solder paste from entering between a substrate and a mask even if solder paste for fine pitches is used. Therefore, a mask is allowed to come into contact with the upper surface of a substrate held on a stage, and solder paste is put on the mask, so that a squeegee is moved along the upper surface of the mask. In addition, magnetic material is used for the mask, and an electromagnet is provided on the stage to absorb the mask through the substrate.
As is known in the art, the magnets are disposed on the table to absorb the mask, and the mask is attracted to the substrate side across the substrate by the attraction force of the magnets to allow the mask and the substrate to adhere tightly to each other. However, the thickness of the mask is usually as thin as 20 μm to 100 μm. Thus, it is necessary to make the magnets closer to the mask within a few mm to reliably absorb the mask to the substrate, and the material of a substrate reception plate to support the substrate needs to be configured using a thin plate. If the substrate reception plate is made thinner, the substrate and the substrate reception plate are likely to be absorbed to the mask due to warpage, and thus it is difficult to keep the flatness.
On the other hand, predetermined tension is added to the mask to keep the accuracy of the mask. Therefore, if the mask is absorbed by the magnets disposed on the table, it is difficult to allow the mask and the substrate to completely adhere tightly to each other. If the substrate and the mask do not completely adhere tightly to each other, one hole of the mask on which a predetermined hole pattern is formed is filled with two or more balls in some cases when filling the substrate with the balls through the pattern holes of the mask, although each hole is supposed to be filled with one ball. This is a phenomenon called “double ball” or “extra ball” in which a pad on a substrate is filled with extra balls.
Further, when the mask is separated from the substrate, the mask is swiftly separated from the outer periphery portion of the substrate while being bent because the substrate is separated from the mask while keeping the magnetic force. Therefore, the balls mounted on the substrate with difficulty are moved from predetermined positions, and a phenomenon called “shift ball” in which balls are shifted from regular mounting positions occurs. Further, when the mask is swiftly separated, the balls mounted on the substrate are flicked, and the balls disadvantageously adhere to and remain on the side faces of the pattern holes of the mask.
When the mask is absorbed to the substrate by the magnetic force of the magnets attracting the mask, the micro-ball mounting apparatus described in Japanese Patent Application Laid-Open No. 2008-192818 realizes uniform absorption by making the absorption force smaller in the middle portion than in the peripheral portion. However, although the apparatus has a relative difference of the absorption force, the mask is always absorbed, and thus the following troubles are likely to occur. Specifically, when the mask is separated from the substrate after the solder balls are completely mounted on one substrate, the mask is separated while making the absorption force in the middle portion smaller, in other words, while always providing the force of absorbing the mask when viewed from the substrate side. Thus, the bending of the mask is not eliminated on the whole surface. Therefore, under the circumstance in which the outer diameter of solder balls is reduced every year, it is difficult to stably mount the solder balls.
Further, in Japanese Patent Application Laid-Open No. 2012-19000, the electromagnet is embedded on the table to absorb the mask and the substrate. However, if the electromagnet is used, it is necessary to attach plural electromagnets. In addition, a control device provided with a demagnetization function is needed because the magnetic force remains even when the supply of current to the electromagnets is stopped. Further, the electromagnets release heat to the outside due to heat generation, and thus a cooling function is necessary to avoid deformation of the table due to the heat. In addition, the structure of the table becomes complicated and the cost thereof is increased.